I was thinking about suggesting using 2 gaskets. It could be a possibility.
Also, be wary of tightening the bolts too much (or too tightly) because you can snap off the exhaust manifold ears on the manifold flange and then you have another issue to deal with.
Just asking.....
When you are attaching the pipe to the manifold is the pipe already connected to the muffler?
I would suggest that you, first, connect the pipe to the manifold without it being connected to the muffler. That way you can get the pipe onto the manifold nice and straight and you will have "play" to move the pipe around to get it to sit correctly (straight) before bolting it on.
Also, clean up the original pipe flange with a wire brush to get a nice flush fit